全文获取类型
收费全文 | 4179篇 |
免费 | 165篇 |
国内免费 | 37篇 |
专业分类
化学 | 2667篇 |
晶体学 | 9篇 |
力学 | 169篇 |
数学 | 928篇 |
物理学 | 608篇 |
出版年
2023年 | 22篇 |
2022年 | 30篇 |
2021年 | 90篇 |
2020年 | 102篇 |
2019年 | 90篇 |
2018年 | 89篇 |
2017年 | 66篇 |
2016年 | 179篇 |
2015年 | 148篇 |
2014年 | 180篇 |
2013年 | 275篇 |
2012年 | 315篇 |
2011年 | 323篇 |
2010年 | 157篇 |
2009年 | 197篇 |
2008年 | 270篇 |
2007年 | 259篇 |
2006年 | 218篇 |
2005年 | 203篇 |
2004年 | 205篇 |
2003年 | 144篇 |
2002年 | 103篇 |
2001年 | 47篇 |
2000年 | 41篇 |
1999年 | 36篇 |
1998年 | 39篇 |
1997年 | 34篇 |
1996年 | 46篇 |
1995年 | 22篇 |
1994年 | 28篇 |
1993年 | 17篇 |
1992年 | 30篇 |
1991年 | 19篇 |
1990年 | 23篇 |
1989年 | 21篇 |
1988年 | 21篇 |
1987年 | 17篇 |
1986年 | 15篇 |
1985年 | 34篇 |
1984年 | 25篇 |
1983年 | 25篇 |
1982年 | 29篇 |
1981年 | 15篇 |
1980年 | 18篇 |
1979年 | 11篇 |
1978年 | 19篇 |
1977年 | 13篇 |
1976年 | 14篇 |
1974年 | 11篇 |
1967年 | 4篇 |
排序方式: 共有4381条查询结果,搜索用时 359 毫秒
991.
Caballero A Tormos R Espinosa A Velasco MD Tárraga A Miranda MA Molina P 《Organic letters》2004,6(24):4599-4602
A new 1-(9-anthryl)-4-ferrocenyl-2-aza-1,3-butadiene showing selective fluorescence enhancement upon binding to lithium cations over other alkaline cations in aqueous environment has been conveniently synthesized and characterized. [structure: see text] 相似文献
992.
Andrea M. Gasco Clara Cena Antonella Di Stilo Giuseppe Ermondi Claudio Medana Alberto Gasco 《Helvetica chimica acta》1996,79(7):1803-1817
The synthesis, structural characterization, and NO donor properties of a series of terfuroxan (= terfurazan trioxide) derivatives 1a–h and 2a–j are reported (Schemes 1 and 2). Structural assignments were confirmed principally by mass and 13C- and 1H-NMR spectroscopy. The extent and the initial rate of NO release in the presence of thiol cofactor was evaluated for each derivative. Vasodilator effects of all the terfuroxan derivatives were evaluated on endothelium-denuded strips of rat aorta precontracted with noradrenaline. Concentration-response curves were also evaluated in the presence of 10 m?M oxyhemoglobin (HbO2), a well known NO scavenger. The whole series displays high vasodilating potency; the most active terfuroxans ( 1a, b, g and 2i ) are 5–10 times as potent as glyceryl trinitrate taken as reference (see Table 3). The potency decrease observed in the presence of HbO2 agrees with the involvement of NO in the vasorelaxing action. The 4,3′:4′,4″ connection (series 1 ; furoxan numbering) gives rise to the most potent compounds. The conformational factors seem to play important roles in the activity. No clear relationship between physico-chemical properties of the substituents and potencies of derivative emerges. 相似文献
993.
THE PHOTODYNAMIC EFFECT OF ROSE BENGAL ON PROTEINS OF THE MITOCHONDRIAL INNER MEMBRANE 总被引:2,自引:0,他引:2
Cecilia Giulivi Monica Sarcansky Esteban Rosenfeld Alberto Boveris 《Photochemistry and photobiology》1990,52(4):745-751
Photodynamic action promoted by Rose Bengal was evaluated in solutions of unsaturated fatty acids or histidine, and on beef heart submitochondrial particles. Rose Bengal-promoted photooxidation of histidine was mainly due to the opening up of the imidazole ring by singlet oxygen. Photosensitization of polyunsaturated fatty acids (PUFA) resulted in oxygen consumption and thiobarbituric acid-reactive substances (TBARS) formation, the extent of which was linearly related to the increasing degree of unsaturation. Photosensitization of submitochondrial particles caused oxygen consumption and TBARS production. These processes involved two different reaction components: during the first, most of the mitochondrial proteins were inactivated, the most sensitive being succinate dehydrogenase and cytochrome c. The values for the rate ratios of [TBARS] formation/[O2] consumption for the first and second phase were 0.36 and 1.32%, respectively, pointing to a larger contribution of lipid peroxidation during the second phase. The calculation of the rate constants for reaction of singlet oxygen with mitochondrial proteins suggests that singlet oxygen is more reactive towards proteins than to PUFA. The biological role of this selectivity is discussed in terms of the mitochondria as one of the first targets for photosensitized reactions. 相似文献
994.
Alberto R. Dias 《Journal of organometallic chemistry》2005,690(7):1840-1844
Metal-η5-cyclopentadienyl (M-Cp) and metal-η5-pyrrolyl (M-pyr) bond dissociation enthalpies in group 4 complexes were determined from DFT/B3LYP calculations with a VTZP basis set. Thermochemical cycles involving calculated enthalpies of ligand exchange reactions and experimental values of ligand electron affinities and M-Cl bond dissociation enthalpies were applied to [M(η5-X)Cl3] piano stool complexes (M = Ti, Zr, Hf; X = pyr, Cp), allowing a comparative study of those metal-ligand bond strengths. The results indicate that both ligands establish weaker bonds with Ti than with the heavier elements, Zr and Hf. Very similar bond dissociation enthalpies were obtained for pyrrolyl and cyclopentadienyl (within 1 kcal mol−1), suggesting that the well known difference in reactivity between those families of complexes should derive from kinetic rather than thermodynamic causes. 相似文献
995.
Ma Prado García-Aparicio Ignacio Ballesteros Alberto González José Miguel Oliva Mercedes Ballesteros Ma José Negro 《Applied biochemistry and biotechnology》2006,129(1-3):278-288
The influence of the liquid fraction (prehydrolysate) generated during steam-explosion pretreatment (210°C, 15 min) of barley
straw on the enzymatic hydrolysis was determined. Prehydrolysate was analyzed for degradation compounds and sugars' content
and used as a medium for enzymatic hydrolysis tests after pH adjusting to 4.8. Our results show that the presence of the compounds
contained in the prehydrolysate strongly affects the hydrolysis step (a 25% decrease in cellulose conversion compared with
control). Sugars are shown to be more potent inhibitors of enzymatic hydrolysis than degradation products. 相似文献
996.
Analytical results are reported for the determination of inorganic species in water and sediments sampled in the Lamone and Marzeno rivers ecosystem. The species determined are Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Zn, Hg, F-, Cl- Br-, NO3-, SO4--, Na+, K+, Mg++, Ca++, NH4+ in superficial water, and Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Zn, Hg in sediments. For all the elements, in addition to detection limits, precision and accuracy are given: the former, expressed as relative standard deviation (s(r)), and the latter, expressed as relative error (e), were good, being in all cases lower than 6%. 相似文献
997.
Carmen Ortiz Mellet Alberto Moreno Marín Jos L. Jimnez Blanco Jos M. García Fernndez Jos Fuentes 《Tetrahedron: Asymmetry》1994,5(12):2325-2334
Reaction of deoxyisothiocyanato derivatives of 1,2:3,4-di-O-isopropylidene--D-galactopyranose, 1,2:3,5-di-O-isopropylidene--D-glucofuranose, and 2,3:4,5-di-O-isopropylidene-β-D-fructopyranose with ammonia afforded the conesponding sugar thioureas. Both the Z and E stereoisomers around the NH---C(=S) bond were observed in the 1H and 13C NMR spectra of the later in the low temperature range, their relative proportions being a function of the sugar configuration. Experimental evidence for the existence of seven-membered NHO intramolecular hydrogen bonds in the E isomers of thioureas has been obtained from DNMR experiments, rotational barrier height calculations, measurements of temperature coefficients for the 1H chemical shifts of the NH signals, and study of the influence of the solvent polarity in the rotameric ratio. 相似文献
998.
Crestani MG Muñoz-Hernández M Arévalo A Acosta-Ramírez A García JJ 《Journal of the American Chemical Society》2005,127(51):18066-18073
The reactions of the complexes [(dcype)NiH]2, 1, [(dippe)NiH]2, 2, and [(dtbpe)NiH]2, 3, with a mixture of BEt3 and Super-Hydride (LiHBEt3) afforded sigma-borane nickel(0) compounds of the type [(dcype)Ni(sigma-HBEt2)], 4, [(dippe)Ni(sigma-HBEt2)], 5, [(dtbpe)Ni(sigma-HBEt2)], 6, respectively, with the concomitant formation in each case of [(dcype)2Ni2)(H)3][BEt4], 7, [(dippe)2Ni2(H)3][BEt4], 8 and [(dtbpe)2Ni2(H)3][BEt4], 9, respectively. X-ray crystal structures are reported for 4 and 8.The reaction of BEt3 and LiHBEt3 was also reviewed in detail. 相似文献
999.
The 2′-cyclopalladated imine complex , reacts with CO in MeOH to afford the 2′-substituted aryl imine 2′-CO2CH3-5′-OCH3? C6H3CH?NTol (Tol = C6H4-4-CH3). The product of this reaction can be altered by changing the bridging ligand from AcO to Cl, in which case only the 5-membered ring heterocyclic compound is obtained. [Pd(μ-OAc)( 1a )]2 with 2 equiv. of Ph3P and CO (1 atm) gives the heterocyclic which arises from two CO insertion reactions, whereas [PdX( 1a )]2 (X = AcO, Cl) with 4 equiv. of C?NBut and 4 equiv. of Ph2PCH2CH2PPh2 affords the heterocyclic ketenimine [PdCl( 1a )]2 reacts with CH2?CHCO2CH2CH3 to afford 2′(? CH?CHCO2CH2CH3)-5′-OCH3C6H3CHO, and [Pd(μ-OAc)( 1a )]2 with I2 to give 2′-I-5′-OCH3C6H3CHO. Excess CH3O2CC?CCO2CH3 reacts with various substituted cyclopalladated Schiff's bases in MeOH to afford which we formulate as possessing two Pd? C bonds, and one coordinated ester O atom. The X-ray crystal structure of [Pd(μ-OAc)( 1a )]2 has been determined; relevant bond lengths [Å] and bond angles [°] are: Pd? O(1), 2.139(6), Pd? O(2), 2.026(6), Pd? N, 2.039(6), Pd? C(2′), 1.951(8), Pd? Pd, 3.113(1), N? Pd? C(2′), 80.9(3), N? Pd? O(1), 97.5(2), C(2′)? Pd? O(2), 91.7(3), O(1)? Pd? O(2), 89.2(2). 相似文献
1000.
Merino G Méndez-Rojas MA Beltrán HI Corminboeuf C Heine T Vela A 《Journal of the American Chemical Society》2004,126(49):16160-16169
A series of molecules, based on the smallest carbon cluster with one planar tetracoordinate carbon atom, C5(2-), are presented. To gain a better understanding about which electronic factors contribute to their stabilization, several global reactivity indexes, molecular scalar fields, and magnetic responses were calculated. The optimized bond lengths and the topological analysis of the electron density show that the central carbon atom in the parent dianion C5(2-) has a planar local environment, and it is coordinated to four other carbon atoms. The bonding of the parent dianion with the metal cations is highly ionic. The magnetic properties show that the C5(2-) derivatives are strongly diatropic and have a remarkable transferability of structural and electronic features from the anion to the salts. The theoretical analysis suggests that the lithium salt, C5Li2, is the most plausible candidate for experimental detection. 相似文献